Co3ZnC core–shell heterostructures with enhanced electromagnetic wave attenuation ability

Accepted Manuscript ZnO @ N-doped porous carbon/ Co3ZnC core-shell heterostructures with enhanced electromagnetic wave attenuation ability Wei Feng, Yaming Wang, Yongchun Zou, Junchen Chen, Dechang Jia, Yu Zhou PII: DOI: Reference:

S1385-8947(18)30291-2 https://doi.org/10.1016/j.cej.2018.02.078 CEJ 18557

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 December 2017 11 February 2018 17 February 2018

Please cite this article as: W. Feng, Y. Wang, Y. Zou, J. Chen, D. Jia, Y. Zhou, ZnO @ N-doped porous carbon/ Co3ZnC core-shell heterostructures with enhanced electromagnetic wave attenuation ability, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.02.078

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ZnO @ N-doped porous carbon/ Co3ZnC core-shell heterostructures with enhanced electromagnetic wave attenuation ability Wei Feng1,2.3, Yaming Wang*1,2, Yongchun Zou1,2, Junchen Chen1,2 , Dechang Jia1,2, Yu Zhou1,2 1

Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin, 150001, China

2

Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing

Technology, Harbin Institute of Technology, Harbin, 150001, China 3

Department of Materials and Engineering, National University of Singapore, 119796, Singapore

*corresponding author: [email protected] Abstract: This study develops a synthetic strategy to rapidly construct a ZnO core and N-doped porous carbon with embedded Co3ZnC nanoparticles shell heterostructure for electromagnetic wave absorption. ZnO colloidal is used as template and zinc source to fabricate ZIF67@ZIF8@ZnO core-shell structures as precursors. After annealing, the double-layered ZIF coating transforms into N-doped porous carbon and dispersive Co 3ZnC nanoparticles shell outside the ZnO core. Compared with pristine ZnO colloidal and pure ZIF67 derived porous carbon/Co nanoparticles composites, the core-shell heterostructures shows the most prominent electromagnetic wave absorption properties with strong absorption (minimum reflection loss of -62.9 dB) and broad effective absorption bandwidth (5.5 GHz). The enhanced properties can be ascribed to the multiple interfaces, doped atoms and groups, as well as the improved impedance matching endowed by the unique core-shell structure. Keyword: core-shell, MOF, porous carbon, electromagnetic wave absorption

1. Introduction In the past decade, core-shell micro/nano particles have been attracting much attention due to their 1

widespread potential applications in many fields such as catalysis, energy storage, optics and biomedicine [1-5]. It is an effective route to establish multifunctional heterostructures and improve the properties such as dispersity, reactivity, thermal stability and other functionalities of the components by constructing core-shell structure[1]. For example, bimetallic core-shell nanoparticles such as Au@Pd[6], Au@Ag[7], and Au@Pt[8] have much better catalytic activity than the single metallic nanoparticles. Besides, silica coatings are applied on magnetic particles [9, 10] to improve their suspension stability and reduce their temperature-dependent magnetic susceptibility. Magnetic paricles[3] coated with noble metal such as Au have better biocompatibility and activity for optical imaging. Among different kinds of core-shell particles, carbon coated metal/ metallic compound particles occupy an important position in many fields. Carbon coatings on Fe3O4[11] or LiFeO4[12] used for lithium ion battery anode materials can maintain the integrity of electrode and improve the electrical conductivity. TiO2@Carbon core-shell particles show better adsorption capability and photocatalytic activity than single TiO2[13]. Metal nanoparticles coated with carbon materials utilized for biomedical [3, 14] applications exhibit much better dispersion stability and activity than pure metal. In most of these literatures, carbon shells were fabricated through pyrolyzing organic precursor such as glucose, resin and

polymers at high temperature [14, 15]. The precursors coat on the core

usually through molecular adsorption and electrostatic adherence. Besides some investigations prepared carbon coatings by chemical vapor deposition[16, 17] or thermalchemical reaction[5] under rigorous conditions. Electromagnetic (EM) interference resulted from widely application of electronic devices increase

the need of EM absorbing materials [18-21]. EM wave absorbing materials can dissipate EM wave energy rather than merely reflecting them so that it can replace metallic materials to shield EM interference [22-24]. Recently, core-shell particles show great potential in EM absorbing application as they possess various EM wave dissipation mechanisms by combining different kinds of EM absorbers[25-28]. Moreover, the enlarged interfaces and EM wave propagating path in core-shell structured absorber result to stronger absorption and wider absorption bandwidth [27, 29]. Therein, carbon-coated core-shell nanocomposites occupy an important position. Du et al.[30] synthesized Fe3O4@C core-shell composites by pyrolysis of phenolic resin and investigate their thickness-dependent microwave absorption. It was proved that the carbon coating would increase the complex permittivity and improve the characteristic impedance of Fe3O4 microspheres. Liu et al.[17] fabricated Co@C core-shell nanoparticles through arc plasma method for EM wave absorption. The composite combined magnetic and dielectric materials exhibited high absorption intensity and wide absorption bandwidth. Besides, Chen et al.[26] fabricated Fe3O4/carbon core-shell nanorods using glucose as carbon source. The EM wave absorption ability of the composite was also improved due to the effective complementariness between dielectric and magnetic loss. Metal-organic frameworks (MOFs) have gained great attention as a class of nanoporous materials consisting of metal ions and organic ligand, due to their versatility and designable structures [31-33]. They have also been considered as promising precursor to fabricate porous carbon and carbon-based composites. A varies of MOFs such as MOF-5, ZIF67, ZIF-8 and Al-PCP have been utilized to synthesize heteroatoms doped porous carbon for different applications in energy storage[34, 35], gas absorption[36] , catalysis[37] and EM wave absorption[38-40]. However, the researches on

constructing carbon coated heterostructures via MOF-deriving strategies have rarely been reported. By the way of thermal decomposition of ZnO @ MOFs core-shell microspheres, this study constructs a carbon-coated ZnO with Co3ZnC nanoparticles embedded in the carbon matrix heterostructure. The ZnO @ MOFs microspheres are fabricated by rapid in-situ crystallization of Zn-containing ZIF-8 and Co-containing ZIF-67 successively. As the carbonization of ZIFs coating, the diffused Zn atoms reduced from ZnO combine with Co and C to form interstitial alloy Co3ZnC. The dielectric and magnetic properties of the obtained ZnO @ carbon/Co3ZnC microspheres are investigated. Its EM wave absorption property is compared with that of pure ZnO and ZIF-67 derived carbon/Co composites. The mechanism of the enhanced reflection loss for the ZnO@C/Co3ZnC core-shell microshperes is also explained. The synthetic method not only provides a new strategy to synthesize EM wave absorber, but also enlightens the design and fabrication of other core-shell structures especially carbon-coated nanocomposites.

2. Experimental section 2.1 Preparation of ZnO, ZIF67, ZnO@ZIF8 and ZnO@ZIF8@ZIF67 Firstly ZnO colloidal microspheres were fabricated by the method described in previous literatures[41, 42]. Typically, 5.49 g Zn(Ac)2 2H2O and 250 mL diethylene glycol were mixed in a flask to 160℃and the mixture was kept for 1h under reflux. After cooling down to room temperature, the milky product was centrifuged at 9000 r/min and washed by alcohol for three times. The precipitation was dried at 60℃ for 6 h. 0.1 g ZnO colloidal spheres was added into 14 mL methanol and sonicated for 20 min. Then 2.01 g 2-Methylimidazole (2-MIM) was dissolved in 14 mL methanol. The solution was poured into ZnO

dispersion under magnetic stirring and kept stirred for 1h. The white product was centrifuged and washed by methanol for three times. The precipitation was labeled as ZnO@ZIF8. For comparison, other two samples were prepared through the same procedure except for the amount of added 2-MIM. The amount of 2-MIM of the two samples were 1.0 g and 4.02 g, respectively. The washed precipitation without desiccation was added into 20 ml methanol and sonicated for 10 min. Then 0.328 g 2-MIM and 0.291 g Co(NO3)2 were dissolved in 20 mL and 10 mL methanol, respectively. The two solutions were poured into the ZnO@ZIF8 dispersion rapidly under magnetic stirring in succession. Then the reaction mixture was stirred for 6h. After that, the precipitation was collected by centrifugation and washed for three times by methanol. The dried product was named ZnO@ZIF8@ZIF67. Besides, to explore the proper concentration of reactant, two parallel experiments were also carried out as the sample procedures shown above except for the amount of 2-MIM and Co(NO3)2. The corresponding amount are 0.66 g 2-MIM and 0.58g Co(NO3)2 for the first group and 1.31 g 2-MIM and 1.16 g Co(NO3)2 for the second one. ZIF67 was prepared by blending Co(NO3)2 6H2O and 2-MIM solutions. 0.9 g Co(NO3)2 6H2O and 5.5 g 2-MIM were dissolved in 6 mL and 20 mL methanol respectively. Then the two solutions were mixed under magnetic stirring and kept for 6h. The precipitation was collected by centrifugation as above. 2.2 Preparation of NPC, ZnO@NPC and ZnO@BMNPC ZIF67, ZnO@ZIF8, ZnO@ZIF8@ZIF67 were heated to 500 °C and kept for 2h under argon flow of 40mL/min, respectively. The heating rate was 5 °C/min. After cooling down to room temperature, their corresponding products were NPC, ZnO@NPC and ZnO@BMNPC, respectively.

2.3 Characterization The morphologies and structures of the samples were examined by scanning electron microscopy (SEM; Nanolab600i, Helios, 20kV, U.S.A.) and transmission electron microscopy (TEM; TecnaiF2 F30, FEI, 300kV, U.S.A.). Powder X-ray diffraction (XRD) was carried on X-ray diffractomer (XRD, Empyrean, PANalytical, Netherlands) using Cu Kα (λ=1.54 Å) radiation to study the phase composition of the composites before and after thermal treatment. X-ray photoelectron spectra of the composites were required using an X-ray photoelectron spectrometer (XPS, K-Alpha, Thermo Scientific, USA). Raman spectra were measured on a Confocal Raman Microscope (Confocal Raman Microscope, inVia, Renishaw, U.K.) equipped with a He-Ne laser (λ=532nm). Nitrogen adsorption-desorption measurements were carried on a specific surface area analyzers (ASAP-2000， Micromeritics，USA) at 77K. Surface area was calculated by Brunauer-Emmett-Teller (BET) model. Pore size distributions were calculated using Brunauer-Emmett-Teller (BJH) method. The magnetic hysteresis loops were measured using a vibrating sample magnetometer (JDAW-2000C&D, VSM, China). The complex permittivity (εr) and permeability (μr) of the samples were measured using a vector network analyzer (VNA, N5230A, Agilent, U.S.A.) at 2-18 GHz band. The samples were mixed with molten paraffin and compressed to standard rings with outer diameter of 7mm, inner diameter of 3mm and thickness of 3mm to carry on the measurement. Microwave absorption properties were calculated according to the transmission line theory using Matlab 2015 software.

3. Results and discussion The preparation processes illustration was shown in Figure 1. Firstly, ZnO nanocrystals assembled into ZnO colloidal in hot solvent. After adding 2-MIM solution into ZnO dispersion, ZIF 8 crystals

grew on the ZnO colloidal spheres in-situ owing to the supply of Zn2+ from dissolved ZnO in alkaline solution. Then, with the addition of 2-MIM and Co2+ into ZnO@ZIF8 microspheres dispersion, ZIF67 could nucleate and grow on the ZIF8 coating spontaneously due to exactly the same crystal structures of ZIF 8 and ZIF 67. After thermal treatment, ZIF67/ZIF8 coatings on ZnO cores decomposed.ZIF67 transforms into carbon and ZnO nanocrystals and ZIF8 decomposed into carbon and Co which reacts with C and Zn diffusing from ZIF67 layer and ZnO core to further form into Co3ZnC alloy.

Figure 1 Preparation processes illustration of ZnO@BMNPC sample

The monodispersed ZnO microspheres with diameter ranging from 300-500 nm are shown in Figure S1. The surface of the ZnO colloids is rough because they are actually aggregation of a large number of nanoparticles. The morphologies of ZnO@ZIF8, and ZnO@ZIF8@ZIF67 are shown in Figure 2(a) and (b). After being coated with ZIF8, the size of the microspheres enhances significantly and the surface becomes rougher due to the growth of ZIF8 shells. No ZIF8 polyhedrons can be observed in Figure 2b owing to proper ratio of ZnO and 2-MIM. As shown in Figure S2, over high or insufficient 2-MIM concentration will lead to the formation of many ZIF8 polyhedrons as pointed by

the arrrows, which might reduce the homogeneity of the product. ZnO@ZIF8@ZIF67 microspheres are much larger and coarser than its precursor ZnO@ZIF8 due to the formation of ZIF67 coatings. Identically, there is no ZIF67 polyhedron among the core-shell spheres showing the uniformity of the products. To explore the proper concentration of reactants, different amount of 2MIM and Co(NO3)2 6H2O are added into the ZnO@ZIF8 suspensions. As shown in Figure S3, overmuch 2MIM and Co(NO3)2 6H2O will break up the microspheres and result to irregular crystals. Besides, SEM image of ZIF67 in Figure S4 (a) shows typical polyhedrons with diameter around 500 nm. TEM images of ZnO@ZIF8 and ZnO@ZIF8@ZIF67 microspheres in Figure 2 (e) and (f) manifest their typical core-shell structures. Besides, the images show that the outer MOF coatings of the two samples are 45-50 nm and 65-70 nm respectively, suggesting the increased coating thickness of ZnO@ZIF8@ZIF67. The selected area electron diffraction pattern of ZnO@ZIF8@ZIF67 in Figure 2(i) displays a series of concentric circles which can be assigned to ZnO nanocrystals. Besides, the elemental mappings of ZnO@ZIF8@ZIF67 in Figure 2(j) also prove the existence of double-layer MOF coatings on ZnO core. Co element has a more narrow distribution than C in the outer layer of the coating. Zn element concentrates in the core while distributes few in the inner layer of the coating. ZnO@NPC, as the annealing product of ZnO@ZIF8, shows smoother surface than its precursor due to pyrolysis of MOF coating as shown in Figure 2(c). The TEM image of ZnO@NPC in Figure 2(g) reveals the ZnO core and carbon shell structure. It can be seen from its corresponding HRTEM in the inserted image in Figure 2(g) that there are some ZnO nanocrystals embedded in carbon shells which originate from the decomposition of ZIF8 coating. ZnO@BMNPC, derived from

ZnO@ZIF8@ZIF67, has granular surface morphology as shown in Figure 2(d). Its corresponding TEM image shows that a lot of nanoparticles disperse in the carbon shell. The HRTEM image of the core-shell interface in Figure 2(k) proves the presence of some nanocrystals with interplanar spacing of 0.21 nm and 0.26 nm, which may be ascribed to the (111) and (110) crystal face of Co 3ZnC, respectively. The magnification of the lattice fringes of Area A, B, C and D are presented in Figure S5. As mentioned in the previous literature[43], Co3ZnC is a kind of interstitial alloys which has typical metallic structures with carbon atoms in the interstitial voids. At the annealing temperature, the existing ZnO can be reduced to metallic Zn which has a low melting point (419.5°C). The Zn atoms diffuse into the outer carbon coating and combine with Co and C atoms to form Co 3ZnC. Besides, the elemental mapping of ZnO@BMNPC in Figure 2(l) demonstrates its core-shell structure by the distribution of Zn and C elements. Co element has a spotted distribution due to the formation of Co3ZnC nanoparticles. SEM and TEM images of ZIF67 derived NPC/Co are shown in Figure S4 (b) and (c). The polyhedrons exhibit some shrinkage after thermal treatment. Co particles disperse in carbon matrix uniformly.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

C

(l)

A

C

Co

C

Co

N

Zn

B

N

Zn

D

Figure 2 SEM and TEM images of ZnO@ZIF8 (a) (e); ZnO@ZIF8@ZIF67 (b) (f); ZnO@NPC(c) (g) with inserted HRTEM of interface; ZnO@BMNPC(d) (h); SAED of ZnO@ZIF8@ZIF67(i); elemental mapping of ZnO@ZIF8@ZIF67(j); HRTEM of ZnO@NPC (k); and elemental mapping of ZnO@BMNPC (l).The XRD

patterns of NPC/Co, ZnO@NPC, ZnO@BMNPC samples before and after thermal treatment are shown in Figure 3(a) and (b). ZnO@ZIF8 and ZnO@ZIF8@ZIF67 both have peaks which can be assigned to ZnO wurtize and ZIF67. The widened peaks of ZnO, ZnO@ZIF8 and ZnO@ZIF8@ZIF67 and the two samples suggest that the ZnO spheres are assembled by nanocrystals. The ZnO peaks become weaker after the formation of ZIF67 coatings. After thermal treatment, the peaks of ZIF67 disappear and the intensity of ZnO improves significantly. ZnO@BMNPC sample has strong diffraction peaks of ZnO (JCPDS 36-1451) and Co3ZnC (JCPDS

29-0524), which is prone to form at appropriate temperature[43]. However, the peaks of Co3ZnC at 41.5, 48.4,71.1 and 85.8° shift toward left compared with the standard patterns (41.9, 48.8, 71.5 and 86.4°) on account of the increased crystal constant. The Zn element combined with Co and C may originate from the reduction of ZnO by carbon at elevated temperature. Besides, the peak intensity of ZnO of ZnO@BMNPC is much lower than ZnO@NPC suggesting that it has thicker carbon shells.

Figure 3 XRD patterns of NPC/Co, ZnO@NPC, ZnO@BMNPC (b) and their precursors (a)

Figure 4 shows the XPS spectra of NPC/Co and ZnO@BMNPC samples. Compared with the XPS survey spectra of NPC/Co and ZnO@BMNPC, the later has obvious extra peaks which can be assigned to Zn[44-46]. The high resolution spectra of C, N and Co were fitted using Gauss-Lorentrz peak shape after performing a Shirley background correction. The C1s peaks of the two samples can be split into four components: the three peaks at 284.6, 286.0 and 288.6 eV are corresponding to C-C, C=O and C-OH bonds [47], while the one at 285.2 eV should be assigned to N-sp2 C bond. The N 1s spectra can be fitted to four sub-peaks at 389.4, 399.3 and 400.7 eV, which are corresponding to pyridinic, pyrrolic and graphitic nitrogen respectively[47, 48]. The Co 2p spectra of the two samples can both be fitted to a series of peaks which mainly correspond to metallic Co, oxidized or carbonized Co and nitrided Co. The peak at 777.9 and the one at 792.9 eV can be assigned to 2p3/2

and 2p1/2 orbital levels of metallic Co. The peaks at 779.1 eV as well as the peak at 796.3 eV belong to 2p3/2 and 2p1/2 orbital levels of Co xOy species, respectively, with the corresponding satellite peaks at 786.2 and 799.6 eV[49].

Figure 4 XPS survey spectra and C, N, Co high resolution XPS spectras of NPC/Co and ZnO@BMNPC

Raman spectra of NPC/Co, ZnO@NPC and ZnO@BMNPC are shown in Figure 5 (a). All the samples exhibit two distinct peaks of carbon at ~1354 cm-1 and ~1591 cm-1, which are named as D peak and G peak respectively. The presence of D peaks can be ascribed to the defects and disorder in carbon nanocrystals. G peaks originate from the in-plane vibration of sp2 carbon atoms. The ratio of the intensity of D peak and G peak can be seen as a measure of disorder degree in graphite material[50]. Higher value of ID/IG usually represents higher disorder degree. The experiments of Ferrari and Roberson[51] show that ID/IG will increase when amorphous carbon transform into nanocrystalline carbon due to the appearance of defects and disorder in nanocrystals. The ID/IG

values of the NPC/Co, ZnO@NPC and ZnO@BMNPC are 0.99, 0.91 and 0.93, respectively, indicating that NPC/Co has the highest graphitization among the three samples due to the absence of ZIF8-derived porous carbon. ZnO@BMNPC has higher graphitization than ZnO@NPC because of the presence of Co elements, which can catalyze the crystallization of carbon. Besides, NPC/Co and ZnO@BMNPC both have peaks at 475 and 681 cm-1 which can be assigned to Eg and A1g Raman active modes of CoOx, and a group of peaks at 199, 523 and 616 cm-1 which correspond to F2g Raman active modes of CoOx[52]. Thus the spectras confirm the presence of oxidized Co in the two samples. However, no vibrations of metallic cobalt could be observed, which accords with the observation in other literatures[53]. As shown in Figure 5(b), NPC/Co sample shows typical ferromagnetic hysteresis loops because of the existence of magnetic Co nanoparticles. In the meantime, although there is no Co particle in it, ZnO@BMNPC also shows hysteresis loops suggesting Co 3ZnC exhibits intrinsic magnetism. However, ZnO@BMNPC has a much lower low saturation magnetization (Ms) value of 2.0 emu g-1, compared with 53.2 emu g-1 of NPC/Co, which is ascribed to its low content of magnetic Co3ZnC. Besides, the remnant magnetization and coercivity of NPC/Co is 10.17 emu g -1 and 142.8 Oe, respectively. And ZnO@BMNPC has rather small remnant magnetization and coercivity of 0.11 emu g-1 and 39.2 Oe, suggesting its smaller particle size of magnetic phase.

Figure 5 Raman spectras of NPC/Co, ZnO@NPC and ZnO @ BMNPC (a); Magnetic hysteresis loops of NPC and ZnO @ BMNPC (b)

The frequency dependence of complex permittivity and permeability of ZnO colloidal, NPC/Co and ZnO@BMNPC are displayed in Figure 6. The real part of complex permittivity (ε’) and permeability (μ’) stand for the storage ability of electrical and magnetic energy[23]. The imaginary parts (ε” and μ”) represent the dissipation capability of electrical and magnetic energy[54]. The ε’ values of NPC/Co and ZnO@BMNPC decrease with the increasing frequency while the value of ZnO barely varies in the frequency range. The ε’’ values of NPC/Co and ZnO@BMNPC enhance gradually between 2 and 10 GHz and then decrease between 10 and 15GHz with increasing frequency. After that, the ε’’ values show a small peak at about 17 GHz. The tendency of ε’’ value of ZnO is totally different from the above two ones. It rises up before 13.5 GHz and then drops until 18GHz. Among the three samples, ZnO@BMNPC has medium values of ε’ and ε’’, with the former ranging from 6 to 10 and the later varying from 2 to 3. NPC/Co has the highest values of ε’ and ε’’ among them whereas ZnO shows rather weak storage and dissipation ability of electrical energy. The ε’-ε” curves of NPC/Co and ZnO@BMNPC in Figure S7 reveals that ZnO@BMNPC has more cole-cole semicircles than its counterpart suggesting its additional polarization process resulting from the construction of core-shell structure.

Figure 6 Complex permittivity and permeability of ZnO, NPC/Co and ZnO@BMNPC

As shown in Figure 6, NPC/Co exhibits the highest and ZnO has the lowest values of μ’ and μ’’ in almost the whole band due to it lacks magnetic components. The μ’ and μ’’ values of NPC/Co and ZnO@BMNPC has slightly decrease as the frequency increases. It shows that the magnetic energy storage and loss ability of the samples decrease with the rising frequency. However, the values of μ’ and μ’’ are close to 1 and 0 respectively, showing the dielectric loss have greater influence on the EM wave absorption properties of NPC/Co and ZnO/BMNPC rather than the magnetic loss. Besides, the curves of the two samples both show a small peak at 12 GHz, which can be ascribed to Fabry-Perot resonance[55]. The reflection loss (RL) of the samples was calculated according to the transmission line theory and metal back panel model[56, 57]. As the following equation shows:

(Equation 5) Z0 is the input characteristic impedance of the free space, Zin is the input characteristic impedance at the interface between the absorber and air expressed as following expression: (Equation 6) μr and εr are the relative complex permittivity and permeability of absorber, f is the frequency of the electromagnetic wave, c is the velocity of light, and d is the thickness of the absorber[58]. The reflection loss (RL) of the paraffin-based samples with 40wt. % mass fraction of ZnO colloids, NPC/Co and ZnO@BMNPC at different thickness in the range of 2-18 GHz are shown in Figure 7. It can be seen from Figure 7(a) that ZnO colloidal has barely EM wave absorption properties in the whole frequency range. NPC/Co derived from pure ZIF67 has fairish EM wave absorption ability with the minimum reflection loss of -18.6 dB and effective bandwidth (band with RL< -10 dB) of 4.3 GHz at thickness of 1.6 mm. ZnO@BMNPC sample with core shell structure has the most prominent EM wave absorption ability. When the sample thickness is 2.2 mm, the reflection loss can reach -62.9 dB with effective bandwidth of 5.5 GHz from 11.1 to 16.6 GHz. For NPC/Co and ZnO@BMNPC samples, the frequency corresponding to minimum reflection loss will shift towards left as the sample thickness increases, due to quarter-wavelength attenuation[59]. Thus the frequency range of effective absorption band can be modulated by changing the thickness of absorber.

Figure 7 Reflection loss in the range of 2-18GHz of ZnO, NPC/Co and ZnO@BMNPC Table 1 shows the carbon-based EM wave absorption composites including those derived from MOF in recent literatures. It can be seen from the table that our composite has prominent EM wave absorption with the most strong absorption (RL= -62.9 dB), desired optimum thickness and reasonable effective bandwidth due to the construction of the unique structures. The factors resulting to the enhancement of EM wave absorption properties can be concluded to these aspects. First of all, the core-shell structure, the dispersed Co 3ZnC nanoparticles in the carbon shell and the mesopores in carbon shell endow the composite multiple interfaces, which increases interfacial polarization (called as Maxwell-Wagner effect) resulting to EM wave attenuation[29]. Briefly, the accumulated charge near the interface of heterostructure can dissipate EM energy under alternating electric field. Secondly, the core-shell structure enables multiple reflection of EM wave, which leads to the

extension of EM wave propagating path. Thirdly, the impedance matching of ZnO@BMNPC has been improved compared to NPC/Co composites due to the existence of ZnO core. It contributes to the incidence of EM wave, making it accessible to dissipate inside the absorber. Fourthly, the doped nitrogen atoms and residual oxygen functional groups can also act as polarized center to attenuate EM wave. Table 1. The electromagnetic wave absorption of carbon-based composites in recent literatures Absorber

Maximum

Optimum

RL<-10dB band

content

RL (dB)

thickness(mm)

width(GHz)

20 wt.%

-50.8

1.9

4.8

[60]

40 wt.%

-35.3

2.5

5.8

[38]

Carbon encapsulated Co

50 wt.%

-52.0

3.0

10.6

[61]

Fe3O4/C core-shell nanorings

29 wt.%

-55.4

1.9

~3.6

[62]

50 wt.%

~ -40.0

1.5

~4.2

[30]

40 wt.%

-22.6

2.0

7.2

[40]

50 wt.%

-21.7

1.2

5.8

[63]

40 wt.%

-62.9

2.2

5.5

--

Absorber Carbon nanospheres MOF-derived porous carbon/Co particles

Fe3O4@carbon core-shell spheres MOF-derived Fe/C nanocube MOF-derived Fe-Co/nanoporous carbon This work

Reference

4. Conclusion ZnO core and N-doped porous carbon shell dispersed with Co3ZnC nanoparticles microspheres were synthesized successfully to pursue better electromagnetic wave absorption properties. ZnO colloidal core and double-layered ZIF shell microspheres were fabricated as precursor through in-situ crystal growth strategy. The concentration of reactant was investigated to obtain uniform core-shell structures. After annealing, double-layered ZIF coating pyrolyzed into N-doped porous carbon shell with embedded dispersive Co3ZnC nanoparticles. The obtained ZnO@N-doped carbon/Co3ZnC core-shell microspheres exhibited enhanced electromagnetic wave absorption ability due to the

construction of unique structures. The multiple interfaces and improved impedance matching made it promising candidate for electromagnetic wave absorber.

Acknowledgements The partial supports from the NSFC grant nos. 51571077, 51371071 and 51321061, National Basic Science Research Program (2012CB933900), the Fundamental Research Funds for the Central Universities (HIT. BRETIII.201202) and the program for New Century Excellent Talents in University of China (NCET-08-0166) are gratefully acknowledged.

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Highlight： 1. A ZnO @ carbon doped with Co3ZnC nanoparticles core-shell structure is fabricated. 2. It is obtained by annealing in-situ growing ZnO@ bilayered MOF core-shell microspheres. 3. It has enhanced microwave absorption property due to its unique microstructure.

Graphical Abstract